Conventionally, ε filters are known as a scheme to reduce noise from image signals with a simple structure. An ε filter is to apply a mean filter to each pixel that is located in an image region subjected to image reduction and that has a pixel value falling within a range determined by a certain threshold ε.
Generally, the noise model is represented as shown in FIG. 22A, by a standard deviation (normal distribution) corresponding to the image signal level. For example, when a mean filter is applied, the pixel value x of the target pixel is corrected by using pixel values falling within the range of x±ε (see FIG. 22B). To obtain a sufficient noise reduction effect, it is important to increase the number of pixel values falling within the range of x±ε. That is, unless the threshold ±ε is given an appropriate value, the ε filter cannot offer a sufficient noise reduction effect or may result in blurring of edges and details of the image.
Patent Literature 1 discloses a structure for determining an appropriate threshold ±ε by controlling the threshold of ε filter according to the chroma or color level of an image signal. Patent Literature 2 discloses a structure for controlling the threshold of ε filter according to a noise model obtained by analyzing input image signal.